(a) Field of the Invention
The present invention rates to a thermosetting resin composition which is used as a clear coating to be coated as a top coating on a base coating by a wet-on-wet coating method.
More particularly, the present invention relates to a coating resin composition superior in acid resistance, storage stability, low temperature curing, film appearance, weatherability, etc., which is suitably used as a clear coating composition in a coating system for automobile bodies wherein a base coating containing a color pigment and a metallic pigment is coated on a substrate, a clear top coating is coated thereon by a wet-on-wet coating method, and simultaneously baking is conducted.
(b) Description of the Prior Art
In recent years, characteristics such as high-quality appearance, excellent durability and the like have been required for coatings used for coating of automobile body. In order to satisfy these requirements, a two-coating method is in wide use wherein a clear coating is applied on a base coating.
As the clear coating, acrylic melamine resin type thermosetting coatings of solvent type are in wide use.
Acrylic melamine resin type coatings, however, have no sufficient resistance to acid rain, give rise to blistering, hazing, rain-induced staining, etc., and have no sufficient durability.
In order to solve-these problems, the present applicant proposed a thermosetting solvent type coating composition which utilizes a curing reaction between carboxylic acid group and epoxy group, in Japanese Patent Application Kokai (Laid-Open) No. 139653/1989.
In the above thermosetting solvent type coating composition, problems of acid rain resistance, durability, appearance, etc. were eliminated; however, there occurs viscosity increase during the storage, and the storage stability is poor.
Coating compositions which utilize a curing reaction between acid group and epoxy group, are also described in U.S. Pat. Nos. 4650718, 4681811, 4703101 and 4764430. These compositions generally have poor Storage stability because the curing reaction proceeds even at room temperature, and actually are handled as a two-pack type and the mixing of two packs is conducted right before the use in coating. Thus, the compositions have had serious problems for use in the body coating line of automobile production, as described below.
That is, in the body coating line of automobile production, coatings are generally stored in tanks controlled at a suitable temperature and are made up as they are consumed. The residence time in tank is ordinarily about one month.
In automobile plants, the production lines are often stopped for various reasons. If the stopping makes the above residence time longer, there occurs a viscosity increase of the coating in tanks or pipes as well as gelation in pipes. If such troubles happen, much time and labor are required for the cleaning of tanks and pipes, which may cause serious damage. Particularly when gelation in pipes occurs, not a few days are required until the restart of production, in some cases.